Solid-state detectors based on active reading matrices, for example composed of amorphous silicon (a-Si) with a scintillator layer coating in front of it have been known for many years. The incident X-ray radiation is converted to visible light in the scintillator layer, is converted to an electrical charge in light-sensitive pixel elements in the reading matrix, and is stored on a position-resolved basis.
Related technologies likewise use an active pixel matrix composed of amorphous silicon, but combined with an X-ray converter (for example selenium), which converts the incident X-ray radiation directly to an electrical charge. This is then stored on a position-resolved basis on an electrode in the reading matrix. The stored charge is then read electronically via an active switching element, is converted to digital signals, and is passed on to an electronic image processing system.
One physical characteristic of amorphous silicon which is critical to the image quality is the existence of deep energy levels (traps) which are filled with an electrical charge during recording of an image. The electrical charge is held in a comparatively stable manner in these traps because the energy level is particularly low, so that it is not possible to read all of the charge contained in the trap during the reading process which follows image recording. In fact, a certain proportion of the signal survives in a latent form in the trap and is only gradually released after the reading process. This can lead to a residual signal still being present when a subsequent recording is made, and also being read in the associated reading process. The most recently recorded X-ray image thus includes a residual signal from the previous recording. Among other phenomena, this results in contours from the previous recording appearing as shadows on the subsequent X-ray image. This effect is referred to as a ghosting artifact.
In order to reduce ghosting artifacts, it is known for a flat board composed of light-emitting diodes to be fitted as a reset light underneath the a-Si plate and for the amorphous silicon thus to be stabilized and homogenized by way of defined emitted light pulses.
A conventional X-ray diagnosis device may have a sensor composed of pixel elements and rear-face illumination by elements which are arranged in a matrix, with the output signal from the sensor being measured, and with the elements of the rear-face illumination, which comprise modules with light-emitting diodes which are arranged in the form of a matrix, being driven individually for homogenization by way of a control apparatus.
However, if any individual light-emitting diodes fail, then, in some cases, this can result in a non-uniform reset distribution, which is then reflected in the digital X-ray record.